Mechanical Relay Guide
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Introduction
Mechanical relays are designed to turn on or turn off the power supplied to other devices using a simple signal from a digital output. They are similar to Solid State Relays except they are less expensive, but are also less sophisticated and have a shorter lifespan. You can use them to control LEDs, heaters, appliances, and generally powered device as long as the power you're switching falls within the limits of the relay you're using.
Phidgets sells boards with multiple relays on them, making it easy to control many separate circuits when used with an Interface Kit (or any device with enough Digital Outputs).
How it works
Mechanical relays use a simple electromagnet to open or close the circuit. When current runs through the input and energizes the coil, it creates a small magnetic field which either pulls the arm of the switch away from the other contact of the switch, or pushes it down to close the switch depending on the how the switch is made. A relay also serves as an isolator, because the control (input) and load (output) ends of the relay are not directly connected. This allows you to protect the device you're using to control the relay from power surges in your application.
- (add diagram)
Basic Use
Controlling a mechanical relay is as easy as turning on an LED. Any Phidgets device with a Digital Output will be able to control a mechanical relay.
Contact Bounce
As with any mechanical switch, relays are susceptible to contact bounce. This means that when the switch closes, the arm can bounce on the contact, causing the load's power to flicker slightly. This usually only matters when the application is detecting when the power signal turns on. For example, a circuit designed to increment a counter every time power is applied to its input could incorrectly interpret a bouncing switch as multiple events. Check the switch primer for information on how to deal with switch bounce. It's worth noting that Solid State Relays don't suffer from contact bounce, because the operate without using moving mechanical parts.
Arcing, Interference, and Sticking
When a mechanical relay opens or closes, and the arm of the switch is very close to the contact, the electric current can arc through the air between the contacts. This arc can cause interference with nearby electrical instruments and sensors.
This arcing can heat up the contacts of the switch enough that they can eventually weld together, causing the relay to stay on permanently, which means your load will be powered constantly. If this imposes safety concerns, you should install a fail-safe of some kind. For example, sauna heaters have a simple mechanical thermal shutdown to protect if control electronics fails.
Prolonging Relay Lifespan
In order to prolong the lifespan of your relay, avoid switching loads of higher voltage or current than the relay is build for, and avoid highly inductive loads, which worsen the effects of contact arcing.
Additionally, avoid rapid switching if possible. In fact, you can think of a mechanical relay's lifespan in terms of number times switched rather than amount of time used.
Arc Suppression
The main cause of failure for mechanical relays is electricity arcing across the contacts. To lengthen the lifespan of your relay, you can add various circuit elements that mitigate arcing.
For DC powered applications, the most effective method of arc suppression is to place a feedback diode across the load (diagram).
For AC powered applications, there are a number of options for arc suppression:
- You can put a snubber across the load terminals. A snubber is a simple circuit consisting of a capacitor and resistor in series. (diagram, digikey)
- Recommended values?
- You can put a Metal Oxide Varistor (MOV) across the load terminals in order to protect the relay from voltage spikes. (diagram, digikey)
- You can put a Transient Voltage Suppressor (TVS) such as a SIDAC (Silicon Diode for Alternating Current) to suppress contact arcing. (diagram, digikey)
- How to choose which of the three above?
Choosing a Relay
I need to switch AC
If you need to switch 110V to 240V AC power coming from your power grid, head over to the Mains Voltage section.
- low voltage - same?
I need to switch DC
The DC mechanical relays we sell will withstand a load of up to 5A. If you need to switch a load with current greater than 5A, you should look at DC Solid State Relays.
Types or Classes
Switch Type
One of the major characteristics of a mechanical relay is the design of the switch inside.
Single Pole, Single Throw (SPST)
This is a simple switch with only one path for the current to follow. The relay is either designed to be normally open or normally closed. If it is normally open, the arm of the switch is held away from the contact with a spring when the relay is off and the electromagnet pulls the arm to make contact and close the switch when the relay is turned on. If it is normally closed, the arm of the switch is held to the contact when the relay is off, and the electromagnet pulls the arm away when the relay is turned on.
Single Pole, Double Throw (SPDT)
This switch has two paths for the current to follow. This type of relay is useful if you want to toggle power between two different loads, as pictured. You can also use a SPDT relay as a single pole, single throw switch, and it can function as normally open or normally closed depending on which pins you connect the load to. (another diagram)
Double Pole, Double Throw (DPDT)
This switch functions the same as a SPDT switch, except there are two of them, but are both controlled by a single input on the relay. You can use this type of switch to simultaneously control two separate circuits.
- Latching relay?
Switching
Relays require a minimum voltage on the control line to close the switch. When placing your relays, keep this in mind. If you want to put your relays a long distance from your control unit you will need to make sure that the voltage at the end of the line is still high enough to properly control the relay. For example, the 3051 board requires a minimum of 4.6V to power the relays, supplied via the 3-pin connector. Additionally, the relays on the 3051 will switch with a minimum voltage of 3.3V on the control lines. So when you are installing your 3051 you would need to make sure that the voltages at the end of your control lines are more than 3.3V and the voltage on the power line is more than 4.6V. So long as those conditions are met, you can make the wires as long as you need them to be. If you are unsure if your wires are too long, you can use a multimeter to measure the voltage at the far end of the wire.
If you plan on rapidly switching, you may have to use delays in your program, since the control signals could be changing faster than the mechanical parts of the relay can react.
Conclusion
- Much the same as the Introduction, but using reference to supplied information to justify the statements.
Products in this Category
Glossary
Switch Type
- SPST - either normally open or normally closed. Simple on/off switch.
- SPDT - 3 pins, has both a NO and NC pin and can function like a SPST switch of either type, or can be used to toggle power between two different loads.
- DPDT - Same as SPDT but there are two controlled by the same relay input. Useful for controlling 2 seperate circuits in a synchronized manner.
- Potential selection criteria for specific switching circuits.
Contact Arcing
- As the switch moves from the "on" position to the "off" position (and vice-versa) there is a moment when the arm of the switch in close enough to the contact that electricity will arc through the air, creating lots of heat and possibly damaging the contact. (Diagram would be useful)
- Arcing can lead to sticking, where the switch welds itself closed. If your relay is always on, it might be stuck.
- Arcing will be more severe with higher power loads or highly inductive loads.
- Arcing can be mitigated by:
- For DC, feedback diode across load
- For AC, Snubber circuit or
- MOV or
- TVS
Switching Speed
- Lifespan of relay is measured in number of contacts, not time in use. Faster switching -> shorter life.
- Switching is audible
Operating Time
- Time it takes for the arm of the switch to connect with the contact after being activated
- If 10ms is too slow, look at SSRs
Maximum (AC/DC) Switching Voltage
- Secondary selection criteria.
Maximum (AC/DC) Switching Current
- Main selection criteria.
- If they need to switch more than 5A DC, or 10A AC, go to SSRs
- Inductive loads- does the same multiplier apply as for SSRs?
Minimum Switching Current
- A certain amount of current is needed on the LOAD side of a relay.
- Oxide films build up on the contacts, so this minimum is required to conduct through them.
- This spec determines whether or not the relay can switch a signal instead of power.
- If you are switching a signal rather than power, contact bounce could be a problem.
